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ASIA: Hypercollision Tectonics 271 Asia: continued collision tectonics Closure of an ocean and subsequent collision of two continents result in an intricate process of thrusting and deformation and a reduction to the cessation of relative convergence between the plates when the buoyancy of the continental crust hinders subduction as a whole into the mantle. Other plates reorganize to take up the motion elsewhere. Like in Oman, the active forces responsible for collision may initiate an oceanic subduction zone at a new continental margin or intraoceanic arc system. Tectonics of Asia shows that the behavior of continents after the collision is complex. The continuing penetration of India into Eurasia drives the growth and evolution of the Tibetan Plateau. The strain is partitioned between shortening absorbed on folds and thrusts and lateral extrusion accommodated on large strike-slip faults. Lateral heterogeneities of crustal strength determine the magnitude and distribution of deformation. Normal faulting expresses coeval east-west extension and north-south shortening in the thick continental crust of Tibet. Plate tectonic setting Due to continuous convergence of the Indian and Eurasian plates, the Himalaya and its adjoining region have been undergoing persistent compression since the beginning of collision 50-70 Ma ago. jpb – Intracontinental deformation in Asia Tectonics-2021 272 Plate movements provided by geophysical methods (namely magnetic anomalies in the Indian Ocean and paleomagnetism, see lecture on Himalaya) indicate over 2000 km intra-continental shortening between stable India and stable Asia since the initial collision. The present-day rate of convergence is about 50 mm/yr. Seismic studies and the rate of southward advance of the foreland basin indicate that only 10 to 25 mm/yr. of shortening are currently taken up within the Himalayan thrust system. The rest of the convergence must be absorbed to the north, within and around the Tibetan Plateau. Therefore, the plateau evolution is intimately related to that of the nearby collision zone. As a corollary, any discussion of the India-Eurasia collision system cannot be complete without addressing the nature and evolution of the Asian regions around the collision zone. Tibet Tibet is the world's most prominent topographic effect of continental deformation. It is the largest and highest plateau on Earth, with nearly 3.106 km2 at an average and uniform elevation of nearly 5000 m. The interior of the plateau is flat with internal drainage, low precipitation, and low erosion rates. It is ringed by high mountain ranges: - Its northern margin follows the Kunlun - Altyn Tagh mountains, where the thin Tibetan lithosphere is thrusting the continental Asian lithosphere; - The High Himalayas, where Tibet is thrusting the Indian continental lithosphere, are its southern border. - The Long Men Shan, where the South China continental lithosphere is underthrusting Tibet, delineate its eastern border. - The Karakoram-Pamir ranges define its western boundary. These bordering mountains stand where the plateau overrides the surrounding, low-elevation sedimentary basins underlain by stable Precambrian cratons: - The Tarim Basin to the northwest - - The Qaidam Basin to the north - The Sichuan Basin to the east. - The Indo-Gangetic Basin to the south Subsidiary mountain ranges, such as the Tien Shan and Altai exceed 4000 m in elevation. Early investigators (18th century) quickly realized and measured the gravitational attraction of large mountain ranges and concluded that a crust thicker than the average lays under mountains. Pendulum deflection smaller than expected suggested that material lighter than the mantle beneath the mountain ranges counterbalances the extra-mass of the mountain ranges above the surface of the surrounding lowlands. From this observation, the geological challenge is to know how crustal thickening has developed the mountain “roots”. Note that mountains in the ocean, on the contrary, are ranges with nearly no crust formed on the mantle, again to emphasize the large effect of different densities and buoyancy between the oceanic and continental lithospheres. jpb – Intracontinental deformation in Asia Tectonics-2021 273 Debates over the mode of formation of the Tibetan plateau essentially concern three fundamental questions: (1) How 1000-2000 km of crustal shortening from collision to now are being accommodated? (2) When did the plateau start to rise? (3) How was its remarkably uniform elevation (ca. 5 km above sea level) attained and sustained? These questions refer to the state of stress within the plateau, as documented by the correlation between elevation and deformation regimes: thrusting, hence horizontal compression is dominant at low elevations around Tibet, whereas N-S grabens, hence horizontal extension deforms the southern part of the Plateau, and strike-slip faulting, combining horizontal compression and extension, deforms the northern Plateau. Shortening/Thickening: conceptual models The India-Eurasia collision zone is the largest region of continental deformation on Earth. How is this non-rigid behavior of a continental plate expressed? The hypotheses advanced to explain shortening and subsequent crustal thickening fall into three major categories: crustal-scale thrusting, homogeneous pure shear, and collage. Thrust tectonics The large amount of thrusting derived from the structural and metamorphic geology of the Himalayas led to slightly different but consistent models: Underthrusting of India beneath Asia Argand suggested in 1924 that elevated areas are due to horizontal underthrusting of more than 1000 km of Indian continent beyond the modern Himalayan mountain front, beneath the entire Tibet Plateau and as far as its northern boundary in the Kunlun-Altyn Tagh fold belt. Before the collision, the surface area of the Indian block was therefore much larger than the present-day Indian Peninsula and formed ‘‘Greater India’’. jpb – Intracontinental deformation in Asia Tectonics-2021 274 Holmes (1965) quantified the elevation of Tibet as the isostatic response of a 60 to 75 km thick crust, assuming that the density difference between crust and mantle is 400-500 kg/m3. To explain the doubling of the continental crustal thickness compared to that of the undeformed, surrounding areas, Holmes accepted that the Indian plate underlies the Tibetan Plateau. Two-stacked crust models inspired by Argand's interpretation involve more or less intracrustal shortening of the leading edges of the Indian and Asian continental blocks. Crustal injection – Channel flow An adaptation of Argand’s model involves northward underthrusting (injection) of the relatively rigid Indian crust into the very weak lower crust of Asia. In some ways, the low viscosity, migmatitic crust envisioned in the “channel flow” model discussed for the Himalayas would offer the required mechanical decoupling between the upper crust of Tibet and the underthrusting Indian lithosphere. Distributed thrusting Intracrustal shortening and thickening result from bivergent thrusting in the leading edges of both India and Asia. Crumpling would be symmetrical about a median suture. jpb – Intracontinental deformation in Asia Tectonics-2021 275 Homogeneous shortening and thickening Alternatively, the Tibetan plateau has been attributed to homogeneous, twice normal thickening of the hot and weak continental lithosphere of Tibet regarded as a non-Newtonian viscous continuum. The Tibetan terranes as part of a hot ductile Asia are shortened and thickened in an accordion fashion in front of the advancing cold and rigid India subcontinent. 50% N-S contraction (ca 1000 km) of the initial area to produce today’s surface of the Tibetan plateau must have been accompanied by as much ductile strain at deeper levels. It is logical, therefore, that folds in Tibet trend predominantly east-west. At shallow levels, shortening was essentially brittle. Symmetrical or asymmetrical crustal stacking wedges are variants of bulk homogeneous thickening in response to the shortening of the Asian crust. Collage The Tibetan plateau exposes at least three major, nearly west-east suture zones. From south to north, these are: - The Yalu-Tsangpo Suture, which separates the Indian Plate to the south from the Lhasa Block to the north. - The Late-Jurassic to Early Cretaceous Banggong-Nujiang suture zone, approximately 300 km north of the Yalu-Tsangpo suture, separates the Lhasa Block to the south from the Qiangtang Block to the north. - The Triassic Jinsha suture separates the Qiangtang Block from the Songpan-Ganze terrane to the north. These three sutures represent relicts of the Neo, Meso, and Paleo-Tethys oceans, respectively. Accordingly, Tibet has been attributed to the successive collisions of several continental blocks, probably originating from the break-up of Gondwana, with Asia. Accretion of continental and/or island-arc-type plates thus contributed to the growth of the Asian margin southwards. Two more Paleozoic sutures exist in Asia, further north. Separate events are correlated with the five “tectonic cycles” that dominated the Phanerozoic orogenic history, namely Caledonian (early Paleozoic), Hercynian (late Paleozoic), Indo-Sinian (early Mesozoic), Yenshanian (late Mesozoic), and Himalayan (Cenozoic). Crustal thickening and plutonism are depicted as having repetitively occurred within the underthrusting southern plates. The model highlights the relative antiquity of the penetrative deformation and metamorphism of many of the rocks of the Tibetan plateau but implies
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